The present invention relates to vehicle suspensions and the control thereof. More particularly, the present invention relates to a method of controlling a vehicle suspension by utilizing controllable dampers to distribute damping forces between front and rear axles.
Several types of active control systems for vehicles have been employed to improve vehicle stability and handling by influencing tire forces in the horizontal (yaw) plane. Examples of such systems are active rear steer, active brake control of individual wheels, active suspensions, and active roll bars.
Vehicle handling response to driver steering inputs is to a large extent determined by the forces between the tires and the road surface. During normal driving, tires remain within linear ranges of operation, where tire lateral forces increase proportionally to tire slip angles. Consequently, at a given speed, vehicle yaw rate is proportional to the steering angle. This linear and consistent response of the vehicle to driver steering inputs may change when tires approach or reach the limit of adhesion, as may happen during emergency handling maneuvers or during driving on slippery roads. In these conditions, vehicle handling characteristics can change quite rapidly from those to which the driver is accustomed and affect the driver""s ability to control the vehicle.
It is generally agreed that in order to improve the probability of a typical driver maintaining control of a vehicle in emergency situations, vehicle handling behavior should remain close to that experienced in the linear range of tire operation. Several types of active control systems have been developed to improve vehicle handling. These systems generally influence the tire forces in the yaw plane, thus producing a corrective yaw moment that forces vehicle response to remain close to the desired response. Tire longitudinal and lateral forces are nonlinear functions of surface coefficient of adhesion, tire longitudinal slip, tire slip angle and normal load. With the exception of the surface coefficient of adhesion, each of these variables can be controlled by an electronically controlled chassis subsystem, thus directly or indirectly affecting tire longitudinal and lateral forces.
Tire lateral forces are nonlinear functions of normal load. More specifically, they exhibit a soft characteristic; that is, they initially increase almost proportionally with normal load, then curve gently, and subsequently either saturate or reach a maximum and start decreasing as the load increases. The maximum lateral force typically occurs at the normal load between 1.4 to 2.5 times the nominal load, depending on the type of tire and the tire slip angle. It is noteworthy that for small slip angles, where tire force characteristics remain a linear function of slip angle, the lateral force is a nonlinear function of the normal load; in fact, in this range the nonlinearity with respect to the normal load is more pronounced than at the limit of adhesion.
During cornering maneuvers tire lateral forces along with the vehicle inertial force form a roll moment, which is balanced by the tire normal forces. As a result, vertical loads on the outside tires are larger than on the inside tires. This is referred to as normal load transfer. During transient maneuvers, the proportion between the roll resisting moments developed by front and rear suspensions depends on the distribution of both roll stiffness and roll damping between front and rear suspensions. As a result, total normal load transfer can be split between the front and rear tires in various proportions. Because of the relationship between the normal and lateral tire forces described above, the lateral force per axle decreases as the normal load transfer increases. This mechanism can be used to affect the balance of lateral forces between front and rear axles, thus affecting the yaw response of the vehicle.
Referring to FIG. 10, during a cornering maneuver, if the normal load transfer of the front axle is xcex94Fz1, then the lateral forces generated by the inside and outside tires are Fy1in and Fy1out, respectively. This yields an average lateral force per tire of Fy1, which is only slightly below the lateral force corresponding to the nominal normal load. Suppose now that the normal load transfer is increased to the value of xcex94Fz2, which results in the lateral forces of Fy2in and Fy2out. The average lateral force per tire is now Fy2, which is significantly less than Fy1. By increasing the normal load transfer, the lateral force per axle was reduced by
xcex94Fyf=2*(Fy1xe2x88x92Fy2).
The same mechanism acts on the rear axle, where the normal load transfer is reduced, to increase the lateral force by xcex94Fyr. As a result, a corrective yaw moment
xcex94Mz=xcex94Fyf*axe2x88x92xcex94Fyr*b
is exerted on the vehicle. In the above equation a and b denote the distances of vehicle center of gravity to the front and rear axles, respectively, and the steering angle is assumed to be small. Thus, by changing the normal load distribution among the four vehicle corners, the yaw response can be affected. The effect described above can be achieved, for example, by increasing the damping of the front suspension and reducing the damping of the rear. This represents an oversteer correction, since the yaw moment imparted on the vehicle reduces the rate of rotation. The opposite damper settings will reduce vehicle tendency to understeer.
The mechanism described above is routinely applied to alter vehicle handling characteristics through passive roll stiffness distribution (anti-roll bars) and has been utilized in actively controlled suspension systems equipped with active roll bars. The prior art, however, does not disclose controllable dampers utilized to affect vehicle yaw response.
Accordingly, it is an object of this invention to provide a new active suspension control system that enhances vehicle stability and handling in fast evasive maneuvers performed close to the limit of adhesion.
It is a further object to provide an improved yaw response of a vehicle through the correction of both oversteer and understeer, especially in transient maneuvers performed on road surfaces that have a high coefficient of adhesion.
It is another object of this invention to provide a system that reduces the effort required by a driver to steer the vehicle while performing emergency handling maneuvers.
It is yet another object of this invention to provide a system that reduces the roll velocity and roll angle of a vehicle during emergency maneuvers.
It is another object of this invention to provide a system that maintains a more consistent vehicle response by reducing variations due to changes in payload, tires or such occurrences as rough roads or an inconsistent surface coefficient of adhesion.
It is another object of this invention to provide a system that has the ability to adjust oversteer and understeer characteristics as a function of speed.
Finally, it is an object of this invention to provide a system that is able to perform the above stated objects and functions in an unobtrusive manner.
The invention is a method for using controllable dampers to improve vehicle responses and stability during severe handling maneuvers. The method derives a total handling damping value for the vehicle and a control ratio of one of the front axle or rear axle roll damping to total roll damping for the vehicle in response to vehicle dynamic variables and further derives therefrom handling damping values for one or more controllable dampers, preferably at least the dampers associated with one of the front axle and the rear axle, and controls the dampers in response to their derived handling damping values. For each controllable damper so controlled, the method preferably blends the handling damping value with a damping value derived from suspension component movement to determine a corner damping command for the controllable damper.